Coupled-cluster and density functional theory studies of the electronic excitation spectra of trans-1,3-butadiene and trans-2-propeniminium.

The electronic excitation spectra of trans-1,3-butadiene (CH(2)=CH-CH=CH(2)) and trans-2-propeniminium (CH(2)=CH-CH=NH(2)(+)) have been studied at several coupled-cluster and time-dependent density functional theory levels using the linear response approach. Systematic studies employing large correlation-consistent basis sets show that approximate singles and doubles coupled-cluster calculations yield excitation energies in good agreement with experiment for all states except for the two lowest excited A(g) states of trans-1,3-butadiene which have significant multiconfigurational character. Time-dependent density functional theory calculations employing the generalized gradient approximation and hybrid functionals yield too low excitation energies in the basis set limit. In trans-1,3-butadiene, increasing the basis set size by augmenting multiple diffuse functions is observed to reduce the high-lying excitation energies with most density functionals. The decrease in the energies is connected to the incorrect asymptotic behavior of the exchange-correlation potential. The results also demonstrate that standard density functionals are not capable of providing excitation energies of sufficient accuracy for experimental assignments.